INSQUE(3) Linux Programmer's Manual INSQUE(3)NAME
insque, remque - insert/remove an item from a queue
SYNOPSIS
#include <search.h>
void insque(void *elem, void *prev);
void remque(void *elem);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
insque(), remque():
_SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
DESCRIPTION
The insque() and remque() functions manipulate doubly-linked lists. Each element in the
list is a structure of which the first two elements are a forward and a backward pointer.
The linked list may be linear (i.e., NULL forward pointer at the end of the list and NULL
backward pointer at the start of the list) or circular.
The insque() function inserts the element pointed to by elem immediately after the element
pointed to by prev.
If the list is linear, then the call insque(elem, NULL) can be used to insert the initial
list element, and the call sets the forward and backward pointers of elem to NULL.
If the list is circular, the caller should ensure that the forward and backward pointers
of the first element are initialized to point to that element, and the prev argument of
the insque() call should also point to the element.
The remque() function removes the element pointed to by elem from the doubly-linked list.
CONFORMING TO
POSIX.1-2001.
NOTES
Traditionally (e.g., SunOS, Linux libc 4 and libc 5), the arguments of these functions
were of type struct qelem *, defined as:
struct qelem {
struct qelem *q_forw;
struct qelem *q_back;
char q_data[1];
};
This is still what you will get if _GNU_SOURCE is defined before including <search.h>.
The location of the prototypes for these functions differs among several versions of UNIX.
The above is the POSIX version. Some systems place them in <string.h>. Linux libc4 and
libc 5 placed them in <stdlib.h>.
BUGS
In glibc 2.4 and earlier, it was not possible to specify prev as NULL. Consequently, to
build a linear list, the caller had to build a list using an initial call that contained
the first two elements of the list, with the forward and backward pointers in each element
suitably initialized.
EXAMPLE
The program below demonstrates the use of insque(). Here is an example run of the pro-
gram:
$ ./a.out -c a b c
Traversing completed list:
a
b
c
That was a circular list
Program source
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <search.h>
struct element {
struct element *forward;
struct element *backward;
char *name;
};
static struct element *
new_element(void)
{
struct element *e;
e = malloc(sizeof(struct element));
if (e == NULL) {
fprintf(stderr, "malloc() failed\n");
exit(EXIT_FAILURE);
}
return e;
}
int
main(int argc, char *argv[])
{
struct element *first, *elem, *prev;
int circular, opt, errfnd;
/* The "-c" command-line option can be used to specify that the
list is circular */
errfnd = 0;
circular = 0;
while ((opt = getopt(argc, argv, "c")) != -1) {
switch (opt) {
case 'c':
circular = 1;
break;
default:
errfnd = 1;
break;
}
}
if (errfnd || optind >= argc) {
fprintf(stderr, "Usage: %s [-c] string...\n", argv[0]);
exit(EXIT_FAILURE);
}
/* Create first element and place it in the linked list */
elem = new_element();
first = elem;
elem->name = argv[optind];
if (circular) {
elem->forward = elem;
elem->backward = elem;
insque(elem, elem);
} else {
insque(elem, NULL);
}
/* Add remaining command-line arguments as list elements */
while (++optind < argc) {
prev = elem;
elem = new_element();
elem->name = argv[optind];
insque(elem, prev);
}
/* Traverse the list from the start, printing element names */
printf("Traversing completed list:\n");
elem = first;
do {
printf(" %s\n", elem->name);
elem = elem->forward;
} while (elem != NULL && elem != first);
if (elem == first)
printf("That was a circular list\n");
exit(EXIT_SUCCESS);
}
COLOPHON
This page is part of release 3.55 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.
2010-09-09 INSQUE(3)